rt2x00: rt2800lib: enable RT3593 support

Support for the RT3593 has been implemented in
the previous changes, so it is safe to mark it
supported in the driver.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>